3. digestion and absorption

• small intestine features for max movement:

  • villi have thin walls for short diffusion pathway
  • villi have lots of capiliaries to help maintain conc gradient
  • villi have muscles which move to mix the contents of the ileum, helps maintain conc gradient
• epithelial cells adaptations for digestion:

  • microvilli increase surface area
  • many mitochondria provide energy for active transport
  • carrier proteins for active transport
  • channel proteins for facilitated diffusion
• amylase produced in small intestine and pancreas

  breaks glycosidic bonds in starch to form maltose
• small intestine produces membrane bound disaccharides

  maltase, sucrase, lactase

  break glycosidic bonds to hydrolyse disaccharides into monosaccharides
• glucose and galactose are actively transported using sodium ions through co-transporter proteins
• fructose is absorbed by facilitated diffusion trough a transporter protein
• cotransport (6)

  • sodium removed from epithelial cell by active transport or the sodium potassium pump
  • into the blood
  • maintaining sodium conc gradient between lumen and epithelial cell
  • via carrier or channel proteins
  • glycose moves into blood
  • by facilitated diffusion
• break down of starch (5)

  • amylase from salivary glands/ pancreas breaks down
  • starch to maltose
  • maltase in small intestine
  • breaks maltose into glucose
  • glucose hydrolysed by hydrolysis of glycosidic bonds
• peptidases are enzymes that breakown proteins in a series of hydrolysis reactions
• there are 3 protease enzymes
• endopeptidases hydrolyse peptide bonds within a large protein to create smaller polypeptide chains

  create more terminal ends for exopeptidases to work on
• exopeptidases hydrolyse terminal peptide bonds to create smaller polypeptide chains
• dipeptidases located on the membrane of epithelial cells

  work on dipeptides to hydrolyse peptide bonds

  creates 2 single amino acids which can be transported through the membrance into the epithelial cell
• facilitated diffusion

  no ATP needed

  through carrier and channel proteins
• active transport

  uses ATP
• simple diffusion

  no ATP or transport protein required
• why exo and endopeptidases together is more efficient

  -endo hydrolyse internal peptide bonds

  -creating more terminal ends and increasing surface area for exopeptidases
• structure of proteins (6)

  • polymer of amino acids
  • joined by peptide bonds
  • formed by condensation reaction
  • primary structure is order of amino acids
  • secondary strucutre is folding of polypeptide chain due to hydrogen bonds into alpha helix or beta pleated sheet
  • tertiary structure is 3d foldinf due to hydrogen, ionic adn disulfide bonds
  • quaternary structure is 2 or more polypeptide chains
• how proteins are digested in the gut (4)

  • endopeptidases break polypeptide into smaller chains
  • by hydrolysing internal peptide bonds
  • exopeptidases hydrolyse terminal peptide bonds
  • dipeptidases hydrolyses dipeptides into amino acids
• lipases hydrolyse ester bonds in triglycerides to form fatty acids and monoglycerides
• lipases are made in the pancreas and secreted into the small intestine
• bile salts produced in liver and stored in gall bladder which releases them into the small intestine
• bile salts help break down large fat globules by emulsifying them into smaller droplets. this speeds up lipases action by increasing surface area of lipids exposed to the enzyme
• digestion of lipids (6)

  • micelles contain bile salts and fatty acids / monoglycerides
  • make fatty acids / monoglycerides more soluble and carry them to the lining of the ilium
  • absorbed by diffusion
  • triglycerides reformed in cells
  • vesicles move to cell membrane
• once lipids are broken down into fatty acids and monoglycerides they stay attached to bile salts and form smaller structures called micelles
• micelles

  • deliver fatty acids, glycerol and monoglycerides to epithelial cells of the ileum for absorption
  • water soluble vesicle formed of fatty acids, glycerol, monoglycerides and bile salts
• advantage of lipid droplet and micelle formation (3)

  • droplets increase surface area for lipase
  • so faster hydrolysis of triglycerides
  • micelles carry fatty acids and glycerol through the membrane of epithelial cell